The present invention relates to an organic thin-film electroluminescent display device for use in display panels such as segment matrix panels, dot matrix panels, etc., a method for driving the same and a method for fabricating the same.
The electroluminescent display device is a light-emitting device based on utilization of electroluminescence of solid fluorescent materials. Inorganic electroluminescent display devices using inorganic materials are presently being applied with regard to backlighting, planar display, etc. of liquid crystal displays. However, the inorganic electroluminescent display device has such disadvantages as necessity for driving with a high ac voltage such as 100 V or more and difficulty in blue light emission and consequent difficulty in full colorization based on three primary colors. In 1987, on the other hand, Eastman Kodak Company proposed an organic thin-film electroluminescent device having an organic thin-film double layer structure of functional separation type, where the thin film of organic materials is functionally divided into two layers, i.e. a hole transport layer and a light-emitting layer, and it was found that an organic thin-film electroluminescent display device based on the proposed electroluminescent device had a high luminescence brightness such as 1000 cd/m.sup.2 or more at a low driving voltage of 10 V or less (see Applied Physics Letters, 51 912 et seq). Since then, research and development of organic thin-film electroluminescent display devices of similar laminated structure using organic materials have been extensively made with a keen desire for development of an organic thin-film electroluminescent display device having a larger display area, a higher resolution and a more uniform luminescence brightness.
One conventional example of an organic thin-film electroluminescent display device having such a laminated structure will be described below, referring to FIGS. 5 and 6.
FIG. 5 is a schematic cross-sectional view of a conventional organic thin-film electroluminescent display device and FIG. 6 is a schematic plan view of the conventional organic thin-film electroluminescent display device, where numeral 101 is a transparent substrate of glass, etc.; 102 hole injection electrodes of ITO (indium tin oxide: tin-containing indium oxide), etc. formed on substrate 101; 111 a hole transport layer consisting of a film of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (which will be hereinafter referred to as TPD), etc., formed on hole injection electrode 102; 112 is a light-emitting layer consisting of a film of 8-hydroxyquinoline aluminum (which will be hereinafter referred to as Alqz), etc., laid on hole transport layer 111; 106 electron injection electrodes consisting of Al-Li alloy, Mg-Ag alloy, etc., laid on light-emitting layer 112; 107 an electrode-driving IC mounted on substrate 101 for driving hole injection electrodes 102 and electron injection electrodes 106; 110 lead wires formed on substrate 101 for connecting hole injection electrodes 102 and electron injection electrodes 106 to electrode-driving IC 107, respectively; 113 light-emitting regions within light-emitting layer 112. In FIG. 5, organic thin film layer 103 is in a double layer structure consisting of hole transport layer 111 and light-emitting layer 112. Preferable materials for hole injection electrodes 102 are materials having a good electroconductivity and a high transparency capable of good transmission of light emitted from light-emitting layer 112. As materials for electron injection electrodes 10, materials having a low work function and a high electroconductivity have been proposed so as to enable easy electron injection into light-emitting layer 112. To give a matrix form luminescence to desired regions of the organic thin-film electroluminescent display device, hole injection electrodes 102 and electron injection electrodes 106 are divided in a plurality of segments patterned by etraight lines crossing at right angles to one another, etc., and a plurality of lead wires 110 are provided for connection to the respective hole injection electrodes 102 and electron injection electrodes 106. When hole injection electrodes 102 and electron injection electrodes 106 of the organic thin-film electroluminescent display device in the foregoing structure are driven by application of a dc voltage thereto from electrode-driving IC 107, light-emitting regions 113 within light-emitting layer 112, i.e. regions corresponding to segments enclosed by hole injection electrodes 102 and electron injection electrodes 106, will emit light. Application of a dc voltage to hole injection electrodes 102 and electron injection electrodes 106 is based on such a system with scanning electrodes called common electrodes on the one hand and data electrodes called segment electrodes on the other hand, in which a voltage is applied to crossing points of these electrodes to emit light there. A predetermined voltage is periodically and sequentially applied to a plurality of common electrode-constituting electrodes, while a voltage corresponding to a signal for image display is applied to a plurality of segment electrode-constituting electrodes.
However, in the conventional organic thin-film electroluminescent display device, hole injection electrodes 102 and lead wires 110 are made of the same materials. For example, in case of using an ITO film for the hole injection electrodes, ohmic loss is largely different from one lead wire to another, depending on distances of lead wires 110 from electrode-driving IC 107 to the respective hole injection electrodes 102 and the respective electron injection electrodes 106, because the ITO film has a resistivity as high as 10.sup.16 .OMEGA.cm. Thus, different quantities of current are input to the individual hole injection electrodes and the individual electron electrodes 106 due to the different ohmic losses, and consequently the conventional organic thin-film electroluminescent display device has such a problem as fluctuations in luminescence brightness in the light-emitting regions.
Furthermore, in case of driving either hole injection electrodes 102 of ITO film, etc. or electron injection electrodes 106 of Al-Li alloy, Mg-Ag alloy, etc. as common electrodes of the conventional organic thin-film electroluminescent display device, the common electrodes are so high in electric resistance that ohmic loss largely differs from one common electrode to another, and thus different quantities of current are input to the individual common electrodes, depending on the difference in ohmic loss. Thus, the conventional organic thin-film electroluminescent device has such a problem as fluctuation in luminescence brightness in the light-emitting regions. It is possible to lower the electric resistance by increasing the thickness of hole injection electrodes 102, but the luminescence brightness will be lowered due to increased thickness of hole injection electrodes 102. On the other hand, an increase in the thickness of electron injection electrodes 106 is not satisfactory for lowering the electric resistance.